As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Optical techniques typically apply an incident beam (often referred to as a probe beam) to a sample and then detect and analyze the reflected energy. This type of inspection and analysis is known as optical metrology and is performed using a range of different optical techniques.
One widely used type of optical metrology is known as photo modulated reflectance or PMR. As shown in FIG. 1A, a typical PMR-type system includes a pump laser and a probe laser. The pump laser intensity is varied to create an intensity-modulated pump beam. The pump beam is projected against the surface of a sample and absorbed causing localized excitation of the sample. As the pump laser is modulated, the localized excitation (and subsequent relaxation) creates a train of thermal and plasma waves within the sample. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the surface reflectivity of the sample. Features and regions below the sample surface that alter the passage of the thermal and plasma waves will therefore alter the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be obtained.
To monitor the surface changes, a probe laser is used to direct a probe beam at a portion of the sample that is excited by the pump laser. The sample reflects the probe beam and a photodetector records the intensity of the reflected probe beam. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation. For most implementations, this is performed using a heterodyne or lock-in detector (See U.S. Pat. No. 5,978,074 and in particular FIG. 2 there for a discussion of such a lock-in amplifier/detector). Devices of this type typically generate separate “in-phase” (I) and “quadrature” (Q) outputs. These outputs are then used to calculate amplitude and phase of the modulated signal using the following equations:Amplitude=√{square root over (I2+Q2)}  (1)Phase=arctan(Q/I)  (2)
The amplitude and phase values are used to deduce physical characteristics of the sample. In most cases, this is done by measuring amplitude values (amplitude is used more commonly than phase) for one or more specially prepared calibration samples, each of which has known physical characteristics. The empirically derived values are used to associate known physical characteristics with corresponding amplitude values. Amplitude values obtained for test samples can then be analyzed by comparison to the amplitude values obtained for the calibration samples.
Characterization of samples using I and Q outputs is described in U.S. patent application Ser. No. 10/387,259, filed Mar. 12, 2003, assigned to the same assignee and incorporated here by reference. In this case, experimentally obtained in-phase and quadrature signals are plotted in I-Q coordinates, analyzed, and compared to calibration data.
The PMR-type system shown in FIG. 1A includes a beam scanner for shifting the X-Y position of the pump beam on the sample surface. This allows the pump beam to be accurately located to coincide with the portion of the sample illuminated by the probe beam. As shown in FIG. 1B, the beam scanner can also be used to separate the pump and probe v beams on the sample surface. As described in U.S. Pat. No. 5,978,074 (incorporated here by reference), this mechanism can be used to scan the separation between pump and probe beams. Amplitude and phase (or I and Q) are measured and analyzed as a function of pump-probe beam separation.
As part of the manufacturing process, ions (or dopants) are added to the near-surface region of semiconductors using a process known as implantation. The implanted region (with its relatively high dopant concentration) overlays a non-implanted region where dopant concentrations are relatively low. The transition between the implanted region and the non-implanted region is commonly referred to as a junction. For advanced semiconductor manufacturing, it is generally desirable for the implanted region to be shallow, typically 500 Å or less. Devices of this type are generally referred to as having ultra-shallow junctions or USJ.
The quality of USJ wafers (and the processes used to create USJ wafers) is typically assessed using two parameters: junction depth and junction abruptness. Junction depth (Xj) is the depth at which the junction between the implanted and non-implanted regions is located. Abruptness (measured in nm/dec) characterizes how quickly the junction transitions between high level and low level dopant concentrations. To illustrate, FIG. 2, shows several USJ profiles obtained by using the Secondary Ion Mass Spectroscopy (SIMS) technique having different junction depths and abruptness (the lower the abruptness value in nm/decade the more abrupt is the profile).
In practice, shallow junction depth is typically accomplished by implantation at high dopant dose and low energy. Abruptness is typically achieved using a rapid thermal annealing (RTA) process. In practice, the required junction depth is often relatively easy to achieve. However, keeping the USJ profile abrupt and close to the surface after anneal is a big challenge. As a result, techniques to measure junction depth and abruptness are critical for the manufacture of USJ semiconductors.
Incompleteness of anneal is another parameter that is crucial to USJ characterization. Incompleteness appears when non-uniformities in structural damage caused by ion implantation along with malfunctioning of the RTA process and other types of annealing processes result in residual structural damage areas on the surface of a semiconductor wafer after anneal. This incomplete anneal should also be monitored to increase manufacturing yield and to ensure high performance characteristics of a semiconductor device.
A number of techniques have been developed to characterize the effectiveness of USJ processes. Destructive and contact techniques include secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and spreading resistance depth profiling (SRP). These methods are capable of providing detailed USJ profile information, but at the expense of a turnaround time that is usually measured in days or even weeks or at the expense of damaging the surface with contacts. Alternately, U.S. patent application Ser. No. 09/799,481 (published as U.S. 2002/0167326) describes a non-destructive method for measuring profile abruptness. According to this method, several amplitude measurements are taken at different powers of the pump laser in a photothermal system similar to that described above. The resulting power dependencies are than fitted to a function (power series) and the second (quadratic) coefficient of that function is correlated to profile abruptness. FIG. 3 shows the resulting calibration dependence relating quadratic coefficient and USJ profile abruptness measured independently using another destructive technique. This method has several disadvantages. First, increasing pump power (up to 100 mW) could result in altering of semiconductor device properties. Secondly, interpretation of experimental results in this method is compromised by strong signal non-linearity arising as a result of excitation with high pump power densities. Finally, this method is time-consuming because of the fitting involved and is not sensitive enough to ensure its reliable performance.